Imaging and Monitoring Temporal Changes of Shallow Seismic Velocities at the Garner Valley Near Anza, California, Following the M7.2 2010 El Mayor-Cucapah Earthquake

Seismograms from 700 local earthquakes recorded at various depths (0, 6, 15, 22, 50, and 150 m) by sensors of the Garner Valley Downhole Array in Southern California are used to analyze the shallow velocity structure and temporal changes of seismic velocities after the 2010 M7.2 El Mayor-Cucapah (EMC) earthquake. The direct P and S wave travel times between surface and borehole stations reveal very low shear wave velocities (178-259 m/s) and very high V-p/V-s ratios (6.2) in the top 22 m. Temporal changes of seismic velocities after the EMC earthquake are estimated using autocorrelations of data in moving time windows at two borehole stations (22 and 50 m) and seismic interferometry between multiple station pairs of the Garner Valley Downhole Array. The S wave velocity in the top 6 m drops abruptly by 14.3 3.3%, during the passage of surface waves from the EMC event with a peak ground acceleration of 39 Gal, and recovers in 236 s. The average velocity reductions decrease with depth and are 10.9 3.1%, 8.5 2.1%, 6.3 2.1%, and 4.5 +/- 2.0% in the top 15, 22, 50, and 150 m, respectively. Comparisons of seismic interferometry results between sensor pairs at 0-22 and 22-150 m indicate that statistically significant velocity changes are limited at the site to the top 22 m. Pore pressure data are in phase with the surface displacement and reach maxima when the highest velocity drop occurs, suggesting fluid effects contribute to the observed velocity reductions.